In a projection video display, geometrical raster distortions result from the physical placement of the cathode ray display tubes. Such raster distortions are exacerbated by the use of cathode ray tubes with curved, concave display surfaces and the inherent magnification in the optical projection path. The projected image is composed of three scanning rasters which are required to be in register one with the other on a viewing screen. The precise overlay of the three projected images requires the adjustment of multiple waveforms to compensate for geometrical distortion and facilitate the superimposition of the three projected images. However, manual alignment of multiple waveforms is labor intensive during manufacturing, and without the use of sophisticated test equipment may restrict the complexity of setup undertaken at a user location. Digital convergence in a projection television receiver can be achieved using a two dimensional matrix of adjustable factors applicable to points distributed at regular intervals across the visible screen area. The degree of deflection correction may be finely adjusted at each of these points independently.
In the horizontal direction the deflection correction is determined by a numeric digital factor applicable at the matrix points, which is converted to an analog signal for driving a convergence correction coil. At intermediate points between the points on the matrix, the correction factor is determined by an averaging filter. In the vertical direction it is necessary to calculate the values for the intervening scan lines between the lines corresponding to the points on the correction matrix.
Green is typically chosen to be centered on the projection system optical axis. In this position, the image on the face of the green tube suffers least geometric distortion hence it is chosen as the geometric reference. Red and blue displays are positioned on the axis vertically but typically are located off the optical axis horizontally. As a result, the red and blue rasters are additionally distorted and require keystone shaping to compensate for this off axis projection location. The green raster is sized and shaped by correction waveforms to minimize geometric distortion. The red and blue rasters are then matched to align and precisely overlay the green image. The uncorrected green raster suffers a large vertical pincushion distortion which is corrected by a correction waveform. For optimum geometry, the correction waveform along each column has a distinct S-shape having sinusoidal and parabolic components, and for the off axis red and blue images an additional linear component is required.
The correction waveform may, for example, be adjustable along a matrix of factors defining 13 rows and 16 columns. For each point in the matrix, numerical factors define the associated displacement of the red, green and blue rasters to be effected at each point to achieve accurate picture geometry and superimposition of the red and blue rasters with the green raster. Convergence errors can occur where the red and green and blue images separate horizontally at the center of the display image and, although user centering capability may be provided, such controls can result in more sever mis-convergence at peripheral image locations. The use of cathode ray tubes having concave spherical phosphor surfaces may exacerbate such peripheral image mis-convergence if the individual rasters are not repositioned to their original nominally circumscribed location on the tube face. Hence it is desirable that a user enabled centering control facilitate the simultaneous movement of all rasters.